The present invention relates to an operating circuit for an electric discharge lamp which can shorten the starting time, restriking time and the stabilization time of the luminous flux, and can improve the overall reliability of the circuit.
Conventionally, there has been known a circuit for operating an electric discharge lamp in which two operating circuits having different load characteristics, respectively, are connected in parallel to operate HID (high intensity discharge) lamp such as a metal halide lamp or xenon lamp by DC power source. For example, in U.S. Pat. No. 3,471,747 and Japanese Utility Model Publication No. 19838/82, there is disclosed a circuit in which an operating circuit for supplying the starting voltage and another operating circuit for lighting an electric discharge lamp at a rated voltage are connected in parallel.
In addition, in Japanese Utility Model Publication No. 35192/82, there is disclosed a circuit for lighting an electric discharge lamp in which a DC power supply for supplying a predetermined constant current to the electric discharge lamp is connected in a series with another DC power supply which supplies a current larger than the predetermined current to the electric discharge lamp when starting the electric discharge lamp, and through which the above-mentioned predetermined current flows when the electric discharge lamp becomes stable. However, none of these circuits disclose a parallel circuit of an operating circuit for starting an electric discharge lamp, an operating circuit for stabilizing the electric discharge lamp in its early stage, and an operating circuit for lighting the electric discharge lamp at a rated voltage.
In electric discharge lamps, such as a metal halide lamp where the starting voltage is high, it takes a long time for the lamp characteristics, for example, a lamp voltage and lamp luminuous flux, to become stable after the lamp is lit. A sufficient amount of high voltage is required for starting the electric discharge lamp, and, at the same time, a sufficiently large current is needed immediately after the lamp is started.
Furthermore, in a conventional circuit in which the operating circuits for applying high voltage are connected in parallel to ignite an electric discharge lamp, the current continues to be supplied from the operating circuits for starting to the electric discharge lamp long after the lamp has started and is stably operating. This is undesirable in terms of the reliability of the circuit.